1. Field of the Invention
The present invention relates to an ink composition suitably used for inkjet recording, an inkjet recording method and, furthermore, a printed material obtained by employing the ink composition and a process for producing a lithographic printing plate.
2. Description of the Related Art
With regard to an image recording method for forming an image on a recording medium such as paper based on an image data signal, there are an electrophotographic system, sublimation type and melt type thermal transfer systems, an inkjet system, etc. In the electrophotographic system, a process of forming an electrostatic latent image on a photosensitive drum by electrically charging and exposing is required, and the system is complicated; as a result, there is the problem that the production cost is high. With regard to the thermal transfer system, although the equipment is inexpensive, due to the use of an ink ribbon there is the problem that the running cost is high and waste material is generated.
On the other hand, with regard to the inkjet system, the equipment is inexpensive and, since an image is formed directly on a recording medium by discharging an ink only on a required image area, the ink can be used efficiently and the running cost is low. Furthermore, there is little noise and it is excellent as an image recording system.
With regard to an ink composition that can be cured by irradiation with radiation such as ultraviolet rays and, in particular, an inkjet recording ink composition (radiation curing type inkjet recording ink), there is a desire for an ink composition that cures with high sensitivity and forms an image with high image quality. By achieving higher sensitivity, high curability upon exposure to actinic radiation can be imparted, and there are therefore provided various benefits such as a reduction in power consumption, longer lifetime of an actinic radiation generator due to a decrease in the load thereon and, as a result of adequate curing being achieved, suppression of evaporation of uncured low molecular weight material and of a reduction in the strength of an image formed.
Furthermore, there is a desire for an ink composition that gives an image (printed material) that is resistant to cracking, peeling off, etc., and gives a cured film that has excellent flexibility and adhesion to a substrate. A cured film having high flexibility and adhesion to a substrate enables a printed material to be displayed or stored for a long period of time in various environments while maintaining high image quality, and also has advantages such as ease of handling of the printed material.
Furthermore, improvement in the image strength due to higher sensitivity imparts high plate life to an image when the ink composition is used for the formation of an image of a lithographic printing plate.
As an ink composition that cures with high sensitivity and gives a cured film having excellent flexibility and adhesion to a substrate, an ink composition comprising an N-vinyllactam has been disclosed (Japanese Registered Patent No. 2880845). However, the ink composition described in this patent publication is a highly viscous ink composition containing a polymer and an oligomer as main ink components, and it is difficult to discharge by ink jet.
Furthermore, as an ink composition that can be cured by irradiation with radiation such as ultraviolet rays, an ink composition having excellent adhesion has been disclosed (Published Japanese translation of PCT application No. 2004-514014).
Conventionally, when a lithographic printing plate is produced, a so-called PS plate having a constitution in which a lipophilic photosensitive resin layer is provided on a hydrophilic support is used; this photosensitive resin layer is imagewise exposed to light to thus improve or degrade the solubility of the exposed area toward an alkaline developer and form an image, and the non-image area is then dissolved and removed. However, in recent years, a digitization technique of electronically processing, storing and outputting image information using a computer has become widespread, and a new image output method that matches the above technique has been desired. In particular, a method that can produce a printing plate without a treatment employing a developer has been examined, and a process for directly producing a lithographic printing plate using an inkjet recording ink composition has been investigated (ref. e.g. JP-A-54-117203; JP-A denotes a Japanese unexamined patent publication application). In this process, an ink is discharged imagewise on the surface of a preferably hydrophilic support by an inkjet method, etc., and this is then cured by irradiation with actinic radiation, thereby giving a printing plate having a desired image (preferably a hydrophobic image). In order to form an image area of a lithographic printing plate, it is desirable that ink droplets discharged onto a support cure quickly without spreading, and there is currently a desire for an ink composition that is suitable for such an application.